The neuron as an analog NOT digital system

On 8 Aug 1995, Marc Anton wrote:
> Though we tend to think of the neuron as essentially
> a digital processor (with its on and off firings) it's really
> closer to an analog processor with infinitely complex chemical
> and electrical functions above and beyond anything capable with
> digital reductionism.
I don't think anyone involved in neuroscience treats real neurons as
purely digital processors, as you suggest. There are at least two
reasons why you may have gotten this impression:
1. Action potentials (neuron "spikes") are indeed all-or-none over most
connections. They are transmitted as a regenerative wave of potential.
If the signal is too weak, it will quickly die and not travel far at
all. If it is just strong enough, it will be regenerated at each point
and will propagate to the synapse. If the signal was very strong, the
result is exactly the same. (There can be a *little* variance in action
potential amplitude and shape, but not much.) However, the real
processing in neurons takes place in the dendrites and soma, where inputs
from various synapses at various times are combined. This is a complex,
nonlinear, and analog function of space and time, and this is widely
recognized.
2. Some researchers work with abstract "artificial neural networks" whose
structure and operation are inspired by real neurons. Some of these may
be digital (indeed, they are almost always implemented on a digital
computer). But these are only cartoon sketches of real neurons, leaving
out nearly all the complexity and detail found in a biological system,
while retaining the bare minimum of properties capable of supporting the
complex behavior of interest. And again, I think any researcher in the
field would readily acknowledge this.
,------------------------------------------------------------------.
| Joseph J. Strout Department of Neuroscience, UCSD |
|jstrout at ucsd.eduhttp://sdcc3.ucsd.edu/~jstrout/ |
`------------------------------------------------------------------'